He could have saved himself some time by looking at two different channels -- assuming the transmitters were in different locations, this would have given him two different ellipses which only intersected at a few points.
In the UK, all channels are typically transmitted from all transmitters (with some regional programming on some channels) and except for some locations where there's too much spectrum overlap and some channels are missing. Right now, they're in transition to digital (DVB-T) so that's probably not quite true anymore, as some transmitters don't have analog channels anymore. Transmitters are usually also too far apart to be able to receive a good enough signal from more than one.
Google "Canary Wharf TV reception" and you'll see that this is not unusual. Those buildings really messed up reception.
Is digital TV immune to these analogue problems?
We actually have it worse in the part of North London where I live - we're in a dip, behind a hill, and don't have line of sight on the digital/freeview transmitters. Everyone has cable.
DVB-T uses QAM, which is susceptible to phase problems. However, given that the information is digital, a decent tuner stands a much better chance of filtering out the weaker component. You definitely won't see ghost images: either your bit-error rate is too high for the built-in redundant coding, or it isn't. So either you get MPEG decoding errors, or you don't.
Digital TV streams have error correction so you are likely to experience either a normal picture or none at all. When I was growing up, even a slightly fuzzy ghosted Analog picture was better than no picture at all.
"So there it is. My television is now an effective Canary Wharf detecting radar. Probably the most interesting thing I've seen on TV for a long time." :)
The physical phenomenon is called multipath and can become an incredibly difficult and important problem in many applications (e.g. radar guided missiles).
Actually, passive radar's getting popular for those who don't want their radar systems bombed out in the first attack run. Specifically, if you don't want stealths coming in & bombing out your radar installations, hide sensitive receivers throughout your city and let the cell towers, tv stations, etc. do the transmission for you.
Donno anything about the signal analysis part of this, but that's the gist of what I heard.
Isn't analog TV transmitted on an FM carrier signal? An FM receiver receiving two signals of the same frequency should synchronize on one, or possibly oscillate between the two. So it seems like his antenna is placed exactly where the two signals are in phase, making them indistinguishable to the receiver. Maybe he could put the signals out of phase by moving his antenna to the other end of the roof?
Which is probably I enjoyed living about 30 yards from that point so much :) Coincidentally, my apartment (flat) was directly above a Sam Smiths pub. Happy days ...
The punchline's very good: "My television is now an effective Canary Wharf detecting radar. Probably the most interesting thing I've seen on TV for a long time."
Most people already have a digital TV receiver, either via terrestrial (Freeview) or satellite (Sky). There's also cable in some areas.
The old analogue terrestrial signals are being turned off region-by-region. Part of the Scottish Borders is already off, with the next switchovers being in August and November in Wales and "Granada", the anachronistic name for the TV region roughly covering the North West of England. Everywhere will be changed over by 2012.
Our system is quite different to that in the US; the four main channels (BBC1, BBC2, ITV and Channel 4) are carried universally by every transmitter, even on relays covering tiny villages of a few tens of people. Rather than getting multiple independent TV stations to change over by a certain date, we just need to go round each region and upgrade all the transmitters to carry four digital multiplexes rather than four analogue channels. (Of course, it's more complicated than this, but that's the gist of what's happening.)
I didn't realize that still went on. I remember reading a long time ago about license inspectors driving around in cars with a directional antenna on the roof looking for unlicensed TVs. I thought that went out in the 60s or so.
The model does explain those painfully dry sitcoms from the 70s and 80s that our PBS imports from you. :)
